This invention relates generally to a gamma correction circuit of a color television camera apparatus and particularly to an improvement whereby the gamma correction value can be adjusted stably and accurately.
Conventional broadcast color television cameras most commonly are the three-pickup tube type in which separate pickup tubes are used for each primary color, red, green and blue. The special feature of this type of camera is the excellent color reproduction and picture quality. The reason for this is that it is possible to provide a gamma correction circuit for the video signal processing circuit for each primary color, and it is possible to independently adjust the correction amount for each circuit.
A circuit constructed as shown in Japanese Utility Model No. 1111299 (KOKOKU 50-14403) is used to make the gamma correction circuit adjustable from the outside. These gamma correction circuits are constructed as fixed and variable. The fixed gamma correction circuit provides the maximum amount of gamma correction of the input video signal. The variable correction circuit is a mixing circuit which can vary the mixing ratio by varying the voltage (DC) controlling the gamma correction from the outside. This mixing circuit generally comprises a double balanced modulator type differential circuit which always outputs a constant value. In other words, this variable gamma correction circuit mixes an uncorrected signal with a signal that has been corrected to the maximum by the fixed gamma correction circuit. By varying the level of the gamma correction control voltage supplied to the variable gamma correction circuit, it is possible to vary the mixing ratio of the two signals and, accordingly, by adjusting the voltage level, it is possible to vary the amount of gamma correction.
However, with this kind of gamma correction circuit in which the above correction value adjustment means is used, drift (mostly temperature drift) occurs between transistors comprising the differential circuit of the variable correction circuit. When this drift occurs, the mixing ratio of the two signals varies, causing changes in the gamma correction amount. Even if such drift did not occur, with the above three-pickup tube type color television camera, variable correction circuits are provided for the video signal processing circuit of each primary color, resulting in variations in the characteristics of the transistors that comprise the different differential circuits so even if, for example, a uniform gamma correction control voltage is applied to each variable correction circuit, fluctuation would result between the maximum correction gamma signal and the uncorrected signal. This in turn would result in differences in the amount of correction provided by each circuit. Similarly, even if each correction circuit is of the fixed type, differences in gamma correction characteristics occur so the correction amount will vary even more.
Conventional color television cameras often use computers to automatically adjust the gamma correction amount. In this case the value of the gamma correction control voltage is digitally processed and only converted to an analog signal in the last stage. It is possible to display the numerical value of this digital value within the variable range of the gamma correction control voltage (using a CRT, LED or liquid crystal display to show the voltage value or percentage). However, the adjustment itself is analog controlled so regardless of how invariable the digital value of the control voltage is, the stability of the gamma correction is based on the stability of the differential circuit, resulting in errors because of the differences in characteristics between the gamma correction circuits of the primary colors. Accordingly, even if the gamma correction control voltage is shown in numerical values, the numerical value is no more than an estimate.